Integrated circuits often require supply voltages of greater potential than that provided by an external voltage source. Memory circuits such as dynamic random access memories (DRAMs) and video DRAMs require higher internal voltages to pre-charge memory word lines and the like. Integrated circuits which are used in systems dependent upon a limited external power supply, such as a battery, must generate the additional supply voltages using conversion circuitry. Charge pumps are known in the art as on-chip voltage generators capable of providing a voltage more positive than the most positive external supply or more negative than the most negative external supply. New generation integrated circuits used in densely populated devices, such as portable computers and phones, require low voltage operation and reduced power consumption.
FIG. 1 is a typical prior art charge pump diagram. Charge pump 100 comprises a control circuit 102, an oscillator circuit 204, and a pump core 106, coupled in series. A typical charge pump such as pump 100 operates in two different voltage domains: a low supply voltage domain (called a Vcc domain herein) and a high supply voltage domain (called a Vccx domain herein). The control circuit decodes pump core runaway and float mode trim settings (discussed later) from tmfzVccp<0:2> (e.g., in Vcc domain) and uses them to control an oscillator enable signal (e.g., in Vccx domain) that is provided to oscillator 104. Accordingly, the sense circuit 102 includes a level shifter (not shown). The oscillator generates an oscillating signal that is provided to the pump core 106 to generate the charge pump voltage Vccp. The pump core generates a current to charge a power bus. The runaway mode trim setting allows the charge pump to be turned on despite the Vccp level, and the float mode trim setting allows the charge pump to be shut off despite the Vccp level.
A level shifter has a weakness when the Vccx domain voltage and the Vcc domain voltage get to levels that are outside of the specification for those voltages. When this happens, the oscillator may act erratically, randomly turning the pump on or off, and potentially elevating the pump voltage Vccp to a level higher than its target voltage. This typically happens in a power-up or power-down situation, or when a mobile device, for example, is entering a deep power down mode. A high Vccp voltage can cause reliability issues. For example, when a device powers down or enters a deep power down mode, the Vcc and Vccx signals initially float, then ramp down to 0. Depending upon the amount of leakage on a power bus, the Vcc and Vccx signals may drop at different rates. When the voltages ramp down at different rates, the level shifter used in association with generating the signal for turning the pump core on or off may malfunction if the Vccx level is still above a trip point when the Vcc level drops below the trip point. In this situation, the oscillator circuit may still oscillate, and the pump core may pump, pushing the pump voltage above its target voltage.
In a typical charge pump, a comparator is used to compare the pump voltage to a reference level, to turn the oscillator on or off depending upon the compared levels. The output of the comparator is used with another signal to generate runaway and float signals. If the level shifter malfunctions, the oscillator could continue to run when it is supposed to be shut off
In prior art designs, a power up level translator (PULT) signal generation circuit is used to turn off the oscillator when a malfunctioning level shifter is detected. The PULT typically consumes a steady amount of DC current since it is always on. With the increased use of mobile devices, a PULT solution is undesirable. Still further, a PULT takes a relatively large amount of space on a die. Also, if the PULT does not trip before the level shifter trip point, it becomes ineffective. Alternatively, a resistor based level shifter may be used on the oscillator. This takes up even more die space, and also consumes DC current, but is reliably operated.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved control of runaway and float modes in charge pumps.